[Paper Review] Reconfigurable Intelligent Surface-Empowered Code Index Modulation for High-Rate SISO Systems

Fatih Çögen, Burak Ahmet Özden+2 more|arXiv (Cornell University)|Nov 26, 2022

Advanced Wireless Communication Technologies4 citations

TL;DR

This paper proposes a Reconfigurable Intelligent Surface-Empowered Code Index Modulation (CIM-RIS) system that enhances spectral and energy efficiency in high-rate SISO systems by leveraging spreading code indices and RIS reflection control. Simulation results show significant SNR gains—up to 19.86 dB over traditional RIS—while achieving higher data rates and lower energy consumption through joint modulation of symbol and code indices via RIS phase shifts.

ABSTRACT

In this study, a novel index modulation based communication system is proposed by combining the recently popular code index modulation-spread spectrum (CIM-SS) and reconfigurable intelligent surface (RIS) techniques. This technique is called CIM-RIS in short. In this proposed system, in addition to the traditional modulated symbols, the spreading code indices also carry data by being embedded in the signal, and the reflection/scattering properties of the signals are voluntarily controlled via the RIS technique. Consequently, the proposed system consumes little energy while transmitting extra bits of information compared to the traditional RIS. Average bit-error error (ABER) analysis of the proposed system is carried out and the system complexity, energy efficiency, and throughput analyses are obtained. Performance analysis of the system is carried out on Rayleigh fading channels for the M-ary quadrature amplitude modulation (QAM) technique. It has been shown by computer simulations that the CIM-RIS scheme has better error performance, faster data transmission speed, and lower transmission energy, compared to traditional RIS, transmit spatial modulation aided RIS (TSM-RIS) and transmit quadrature spatial modulation based RIS (TQSM-RIS) techniques.

Motivation & Objective

• To address the growing demand for high data-rate, energy-efficient wireless communication in IoT and multimedia applications.
• To overcome the limitations of traditional RIS systems that transmit all bits via high-order QAM, increasing energy and error rate.
• To improve spectral and energy efficiency by exploiting spreading code indices as an additional information domain.
• To integrate reconfigurable intelligent surfaces (RIS) with code index modulation (CIM) for enhanced performance in Rayleigh fading channels.
• To evaluate system complexity, throughput, energy efficiency, and bit error rate (BER) performance under realistic fading conditions.

Proposed method

• The system combines Code Index Modulation (CIM) with Reconfigurable Intelligent Surfaces (RIS), where data bits are split between modulated symbols (M-QAM) and spreading code indices (Walsh-Hadamard codes).
• RIS elements dynamically control signal reflection phases to convey additional information via beamforming, enabling spatial multiplexing without extra power.
• The transmitter sends one active symbol per time slot using a selected spreading code, with the code index carrying part of the data, while RIS phase shifts further modulate the signal to carry more bits.
• The receiver performs joint detection: first estimating the spreading code index via correlation, then demodulating the M-QAM symbol to recover data.
• System performance is analyzed using average bit error rate (ABER), energy efficiency, and throughput metrics over Rayleigh fading channels.
• Simulations compare CIM-RIS against traditional RIS, TSM-RIS, and TQSM-RIS under varying numbers of RIS elements (N=16 to 128) and bit loads (u=8 to 11).

Experimental results

Research questions

• RQ1 How does CIM-RIS improve spectral efficiency and energy efficiency compared to traditional RIS in Rayleigh fading channels?
• RQ2 What is the impact of distributing data bits between symbol modulation and spreading code indices on error performance and system complexity?
• RQ3 How does the number of RIS elements (N) affect the BER and SNR performance of the CIM-RIS system?
• RQ4 How does CIM-RIS compare in performance to TSM-RIS and TQSM-RIS in terms of SNR gain and bit error rate?
• RQ5 What is the trade-off between data rate, energy efficiency, and system complexity in the proposed CIM-RIS architecture?

Key findings

• The CIM-RIS system achieves an SNR gain of 19.86 dB over traditional RIS when N=64 and u=10 bits, demonstrating superior error performance.
• For N=128 and u=8 bits, the CIM-RIS system provides a 14.2 dB SNR gain over traditional RIS, with gains of 119 dB and 124 dB over TQSM-RIS and TSM-RIS, respectively.
• At u=11 bits and N=32, the CIM-RIS system achieves a 112.8 dB SNR gain over TSM-RIS and 117.68 dB over TQSM-RIS, confirming its robustness under high data load.
• Error performance improves with increasing RIS elements (N), and BER decreases as the number of bits carried in the spreading code index increases.
• The system demonstrates significantly lower transmission energy and higher data rates than traditional RIS, TSM-RIS, and TQSM-RIS due to efficient use of code indices and RIS phase shifts.
• The 3D BER performance shows that increasing the number of reflecting elements (N) and spreading code indices (L) enhances reliability, while higher M-QAM order degrades performance.